Beam position and width sensing by scattering

ABSTRACT

Beam position and beam width sensing is accomplished by scattering a small percentage (0.1 percent) of the beam energy. The scattered light is analyzed by an arrangement of three photosensors that compare the energy distribution along different portions of the beam and produce appropriate error signals which are then used to reposition and focus the beam. Scattering is accomplished by means of thin threads disposed across the wavepath.

United States Patent Inventors Peter Kaiser Middletown; Enrique A. J.Marcatill, Rumson, both of J. Appl. No. 805,201 Filed Mar. 7, 1969Patented Nov. 9, I971 Assignee Bell Telephone Laboratories. IncorporatedMurray Hill. Berkeley Heights, NJ.

BEAM POSITION AND WIDTH SENSING BY SCATTERING 9 Claims,9Drawing Figs.

US. Cl 356/122, 350/96 WG, 356/152, 356/153 Int. Cl G01j1/42, G02b 5/14Field of Search 350/96 WG;356/152, 153, 172. 121, 122.209-211, 199;250/219 WE Christian et al., Self-Aligning Optical Beam Waveguides. IEEEJournal of Quanlom Elcctsonics. Vol. ()Ii 3. 6. June l967,p. 244.

Primary Exuminer-Ronald L. Wibert Arm/ant Examiner-J. RothenbergAuurneys- R. J. Guenther and Arthur I. Torsiglieri ABSTRACT: Beamposition and beam width sensing is accomplished by scattering a smallpercentage (0.1 percent) of the beam energy. The scattered light isanalyzed by an arrangement ofthree photosensors that compare the energydis tribution along different portions of the beam and produceappropriate error signals which are then used to reposition and focusthe beam. Scattering is accomplished by means of thin threads disposedacross the wavepath.

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sum 2 or 4 BEAM BEAM "v- RED R REFOCUSER BEAM LENSES LENSES SENSORSPAIEIIIEIIIIIII 9 RI 3.619066 SHEET l [1F 4 FIG. .9

J I05 I02 I 03 PC REDIRECTING I04 ERROR SIGNAL NETWORK R/zvg s3 4 /g 413 7 S2 Z i R /z /DIFF ERENTIAL DIFFERENTI AL AMPLIFIER AMPLIFIER Eq EbBEAM AXIS BEAM POSITION AND WIDTH SENSING BY SCATTERING This inventionrelates to optical beam position and beamwidth sensing arrangements.

BACKGROUND OF THE INVENTION In his article entitled Effect ofRedirectors, Refocusers and Mode Filters on Light Transmission ThroughAberrated and Misaligned Lenses, published in the Oct. I967 issue of theBell System Technical Journal, pages L733 through L752, E. A. .l.Marcatili calculated the field distortion of a beam propagating througha sequence of identical. misaligned and slightly aberrated lenses. Itwas found that most of the converted power goes to the first and secondhigher order modes. More particularly, it was found that correlated anduncorrelated transverse displacement of the lenses comprising thewaveguide introduce first order mode distortion which causes. in effect,a deflection of the propagating beam. Since this deflection of the beamfrom the desired direction of propagation can grow in proportion to thenumber of lenses. the tolerance requirements imposed upon the lensalignment in an uncompensated system becomes increasingly severe as thenumber oflenses is increased.

It was further found that as long as the lenses are perfect," lensmisalignment only affects the beam direction, without altering thenormal mode beam size or the beam intensity profile. If, on the otherhand, the lenses have aberrations, energy is converted to the secondorder mode and also, to some degree, to still higher order modes whichtogether have the affect of defocusing and distorting the beam. If thisten dency is uncorrected, the location of the beam axis becomes obscureand the ability to redirect the beam is greatly diminished.

It is apparent from the above, that any practical beam transmissionsystem requires some means for regularly sensing both the beam positionand the beam width, and for utilizing this information to redirect andrefocus the beam as required. One such system is described in theabove-identified article by Marcatili.

It is the broad object of the present invention to improve and simplifyoptical beam sensors.

SUMMARY OF THE INVENTION In accordance with the present invention, beamposition and beam width sensing is accomplished by scattering a smallpercentage (0.1 percent) of the incident beam energy. The scatteredenergy is analyzed by an arrangement of photosensitive elements thatcompare the energy distribution along different transverse portions ofthe beam and produce appropriate error signals which are then used toredirect and refocus the beam. Scattering is accomplished by means ofthin threads, diametrically disposed across the beam wavepath.

It is an advantage of the present invention that the beam position andbeam width are determined by sampling directly across the beam profile,rather than by sampling the energy distribution about the edges ofthebeam, as in the prior art.

These and other objects and advantages, the nature of the presentinvention, and its various features, will appear more fully uponconsideration of the various illustrative embodiments now to bedescribed in detail in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows in block diagram anoptical wave transmission system including a beam-correcting system;

FIG. 2 shows a beam sensor in accordance with the present invention;

FIG. 3, included for purposes of explanation, shows the beam scatteringat the beam sensor;

FIG. 4, included for purposes of explanation, shows various possiblebeam orientations;

FIG. 5 shows the variation of beam width between lenses;

FIG. 6 shows. in block diagram, a complete beam sensing and beamcorrection system;

FIG. 7 shows the various elements of the correction system of FIG. 6 ingreater detail;

FIG. 8 shows an intensity profile of an asymmetric beam; and

FIG. 9 shows a four-element photosensor for refocusing and redirectingan asymmetric beam.

DETAILED DESCRIPTION Referring to the drawings, FIG. I shows, in blockdiagram, a typical optical wave transmission system comprising anoptical wave source I0, an optical wave receiver 12 and an opticalwaveguide 11 connecting said source to said receiver. For the reasonsindicated hereinabove, means for sensing the beam position and the beamwidth are advantageously included at regularly spaced intervals alongthe waveguide. This is indicated in FIG. I by a beam sensor 13 locatedalong guide 11.

The output from sensor 13 is coupled to an error signal generator 14wherein a correction signal is developed. The latter is then utilized ina manner to redirect and/or refocus the beam. The present invention isparticularly related to the beam sensor. However, related apparatus forproducing and utilizing the correcting signals will also be consideredin some detail.

FIG, 2 shows a first embodiment of a beam sensor, in accordance with thepresent invention, utilizing beam-scattering techniques.

Recognizing that the position ofa beam can be fully located by referenceto two mutually orthogonal axes, the sensor comprises a pair oforthogonally oriented thin threads 20 and 21 extending diametricallyacross the wave guide enclosure 22. For purposes of identification, thevertically extending thread 20 is referred to as the y-directionscatterer and the horizontally extending thread 21 is referred to as thex-direction scatterer. As an example, the threads can be made of quartz,but any other material which can be drawn to a sufficiently smalldiameter can be utilized.

Associated with the scatterers are the multielement photosensors 25 and26. In this embodiment, each photosensor comprises three longitudinallyextending photosensitive strips located along the inner surface ofenclosure 22. Photosensor 25, which operates in association with thread21 and, hence, is referred to as the x-direction sensor, issymmetrically located with respect to the y-axis. Similarly, photosensor26, which operates in association with thread 20 and, hence, is referredto as the y-direction sensor, is symmetrically located with respect tothe .r-axis.

For purposes of explanation, either the x-or the y-direction sensor canbe considered, it being understood that both operate in the same mannerand independently of each other. Accordingly, in FIG. 3, only the.t-direction sensor, comprising thread 21 and photosensor 25, is shown.

In operation, the incident wave energy impinges upon thread 21. Becausethread 21 is very thin, having a diameter of the order of 0.1 [L to 0.5a, over 99.9 percent of the wave energy propagates past the wire. Asmall fraction, however, of the order of 0.1 percent is intercepted bythe thread and scattered in a plane perpendicular to the thread. Most ofthis energy is scattered in the forward direction with a typical beamwidth of about :30. The scattered energy is intercepted by the threephotosensitive elements comprising photosensor 25. Since the same amountof energy is scattered in the upward direction as in the downwarddirection, the sensitivity of the sensor can be increased by a factor oftwo by mounting an identical set of photoelements diametrically oppositesensor 25. For purposes of explanation, however, only one set ofphotosensitive elements is shown. Since the relative intensitydistribution of the scattered wave energy is essentially the same as theintensity distribution of the incident beam, the scattered energyprovides an accurate picture of the beam profile. Thus, if the incidentbeam has a Gaussian transverse field distribution along the.r-direction. as represented by curve 30, the energy scattered in thedirection of sensor 25 also has a Gaussian field distribution. asrepresented by curve 31. The precise proportion of the scattered waveenergy that is intercepted by each of the three photosensitive elementswill. of course, depend upon its relative cross-sectional dimensions andits location. Assuming, for example, that all three elements have thesame length, the amount ofenergy intercepted by each will vary as afunction of its width. In the special case where all the widths areequal, i.e., W,=W =W the center element will intercept the largestproportion of the scattered energy because of the nature of a Gaussiandistribution. Of particular interest, however, is the case where allthree elements intercept equal portions (i.e., one-third) of the energyscattered from a properly focused, on-axis beam. This preferredcondition is obtained when the width W of the center element is madeequal to 0.432W, where W is the He half-width of the beam. The minimumwidth of each of the two outer strips to intercept an equal amount ofthe scattered energy is then given as W,=W ,=I.784W, where it is assumedthat all the energy is included within a region defined by I 2W.

When proportioned in the manner indicated, a properly focused, on-axisbeam excites all three photosensitive elements of photosensor 25equally. If, on the other hand, the beam, though on-axis, is defocusedthe distribution of energy is disturbed so that proportionately more ofthe energy is scattered onto the two outer elements and less onto thecenter element. This imbalance can then be detected, as will beexplained in greater detail hereinbelow, and used as a means ofgenerating a refocusing error signal. If, on the other hand, the beam isproperly focused but deflected off-axis in either the ir-direction, theenergy scattered onto the two outer elements is unequal, with more ofthe energy received by the outer element towards which the beam isdeflected and less received by the opposite outer element away fromwhich the beam is deflected. This imbalance can also be sensed and usedto generate a redirecting error signal. Clearly, both of thesedeviations from the normal condition can be simultaneously sensed anddetected and both redirecting and refocusing error signals generated, aswill now be considered.

FIGS. 4 and 5, included for purposes of explanation, illustrate thevarious beam conditions that can prevail at the sensor location. Forexample, the axis of a properly oriented beam, represented by arrow 40in FIG. 4, is collinear with the guide axis ZZ. An improperly directedbeam, on the other hand, can be misdirected in one, or both of two ways.It can pass through the guide center at the sensor location but bedirected at an angle A with respect to the guide axis, as indicated byarrow 42, or it can be directed parallel to axis ZZ but displaced adistance d from the guide center, as indicated by arrow 43. Arrow 44represents a beam whose axis is both displaced a distance d from theguide center, and misdirected at an angle A to the guide axis. Thus, inorder to fully determine the beam direction, two parameters must bedetermined. This requires two separate measurements.

Similarly, with respect to the beam focusing two separate measurementsmust be made since a single measurement can only indicate the beam widthbut does not specify whether the beam is narrowing or expanding. This isindicated in FIG. 5, which shows the beam width variations between apair of lenses 50 and 51. As can be seen, the beam width at position 52is the same as the beam width at position 53. However, whereas the beamis narrowing at position 52, it is expanding at position 53.

Thus, in order to fully define the condition of the beam, twomeasurements of the beam orientation, and two measurements of the beamwidth must be made. Accordingly, a complete system will include, asshown in FIG. 6, a pair of beam sensors 60 and 61 longitudinallydisplaced along the guide axis. To insure that the information providedby the two sensors is not redundant, the sensors are located atrelatively different positions with respect to the guide lenses. Thus,in

FIG. 6, one sensor 60 is located immediately adjacent to one of theguide lenses 62, whereas the other sensor 61 is advantageously locatedmidway between lens 62 and the next adjacent lens 63 along the directionof wave propagation, which is assumed to be from left to right.

As explained hereinabove, each sensor provides both beam orientation andbeam width information. Accordingly, one signal from each sensor is fedback to a preceding lens either to reposition the controlled lens, or tochange its focal length. Thus, in FIG. 6, one signal from sensor 60 isfed back to one of the beam redirector lenses 64 and one signal is fedback to a beam refocuser lens 66. Similarly, one signal is fed back fromsensor 61 to beam redirector lens 65 and one signal is fed back to beamrefocuser lens 67.

FIG. 7 shows, in somewhat more detail, the various components of a beamredirector and refocuser system including one of the redirecting lenses71, one of the refocusing lenses 72 and one of the sensors 79. Becausethe present invention is particularly adapted for use in optical systemsemploying thermal gaseous lenses, reference will be made to such lensesin the discussion that follows. Accordingly, the waveguide depicted inFIG. 7 comprises an enclosure 70 within which a transparent gas isflowing, and along which the optical beam propagates. The first thermallens 71, of the type described in U.S. Pat. No. 3,410,627, comprises aheating coil 69 which surrounds the wavepath and which, thereby,establishes a radial density gradient across the flowing gas. Asexplained in the above-identified patent, this has the effect ofproducing lens action. In order to reposition this lens, the lensenclosure is coupled to the rest of the enclosure 70 by means offlexible coupling 68 and 73 which permit transverse displacement of thelens relative to the rest of the system. This displacement is typicallyproduced by a motor 74 which is mechanically coupled to the lens bysuitable gears 75 and a bracket 76.

The second thermal lens 72 is of the type described by Marcatili in hisabove-identified publication. In this lens, the heating member isdivided into four independently controllable heating elements 80, 81, 82and 83. This permits the temperature gradient to be independentlycontrolled along two mutually perpendicular directions and y. Thus, byvarying the temperature of elements 81 and 83, the focusing along the.rdirection is changed, while the v-direction focusing is controlled bythe temperature of heating elements and 82.

The sensor 79 includes an .r'direction scatterer 85 and the associatedphotosensor 86.

Two of the photosensor segments 91 and 92 connect to a first errorsignal generator 93 which compares the two signals produced by theseelements. Assuming the latter to be photoresistors, the resistance ofeach will vary in response to the amount of scattered energy incidentthereon. As a result, the current provided by a series-connected battery98 will also vary, producing signals E and E at the error generator thatare proportional to the scattered wave energy intercepted by photosensorsegments 91 and 92v The generator can be a simple difference amplifierwhose output provides the heater current for elements 81 and 83 of lens72. In particular, with the input signals E and E indicative of properfocusing (i.e., E =E the output current is I(i=0). If, however, therelative amplitudes of E, and E change, an additional heater currentcomponent ii is produced which either increases or decreases the totalheater current, depending upon the relative amplitudes of E and ESignals are also taken from sensor elements and 92 and coupled to asecond error signal generator 94. The latter can also be a differenceamplifier which compares the signals E and E generated at the sensor.When the signals E and E indicate proper beam orientation, the outputsignal from error generator 94 is zero. A change in the relativeamplitudes of signals E and E on the other hand, will produce an outputsignal :1 which drives motor 74 and, thereby, displaces lens 71 in theindirection relative to the rest of the waveguide. The displacementcontinues until the error signal is reduced to zero, indicating that thebeam is centered at the sensor location.

Since. as indicated above. two measurements are required. there would bea second. similar arrangement of beam sensors and lenses. in addition.each sensor would have a v-direction scatterer and associatedphotosensor that would control, in FIG. 7, the heater current to heatingelements 80 and 82, and the redirection signal to a second motor whichwould either displace lens 71 in the v-direction, or control a secondlens.

While the redirection lenses and the refocusing lenses were indicated asbeing separate lenses, it will be understood that lens 72, for example.can just as readily be used for both pur poses. That is, lens 72 canalso be displaced in response to the repositioning signal and, thereby,provide both refocusing and redirecting. It will also be understood thatthe control systems described are merely illustrative. For example,other arrangements, such as that described in the copending applicationby D. H. Ring, Ser. No. 605,741, filed Dec. 29, 1966 and assigned toapplicants assignee. can be used to control the position of theredirector lens.

in addition, it should be noted that many other arrange ments of thephotosensors are possible. For example, as was indicated hereinabove,lens aberrations convert some of the beam wave energy into higher ordermodes. While the typical low level ofsecond order mode wave energy,normally present in a well designated system, tends to defocus the beam,higher order mode wave energy in general tends to distort the beam sothat it is no longer symmetrical with respect to the beam axis. This isillustrated in FIG. 8 wherein curve l00, shown dotted, is the profileofa symmetric beam, whereas curve 101, shown in solid line, is theprofile of an asymmetrically distorted beam.

If an asymmetric beam is applied to the three-element photosensordescribed above, the beam will be refocused and redirected, in themanner described in connection with FIG. 7, so that each of the threephotosensitive elements intercepts equal amounts of the scattered power.The beam, nevertheless, will not be properly centered with respect tothe guide axis. That is, the beam axis, which identifies the regionofmax imum beam intensity, will not be aligned with the guide axis. Thiswould be detrimental during subsequent filtering through, for example, asequence of lenses with small apertures since the losses in such afilter are relatively high if the beam axis does not pass through thelens centers. To avoid these difficulties, the sensor described abovecan be modified so as to insure that the beam axis is properly alignedeven when the beam is distorted. The modification involveslongitudinally subdividing the center element of the above-describedthree-element sensor into two equal portions. The resulting four-elementsensor is represented in H0. 9 which includes a portion ofa guideenclosure 102; two outer photosensitive elements 103 and 104; and twoinner photosensitive elements 105 and 106. In a preferred embodiment,each of the outer elements 103 and 104 are proportioned to interceptone-third of the scattered wave energy. The two inner elements 105 and106 together intercept the remaining one-third. Thus, each interceptsone-half of one-third, or one-sixth of the scattered wave energy.

Repositioning of the beam is accomplished by sensing the signals 8,, andS produced in response to the energy intercepted by sensor elements 105and 106, and redirecting the beam until signal S equals signal 5,. Whenthis condition is satisfied, the beam axis is aligned with the guideaxis. The need for refocusing is indicated when the two differences 5 -8and S have the same polarity. Accordingly, signals S and 5;, are coupledto a first differential amplifier lll) which produces an output signal[5,, which is either positive or negative, depending upon the relativeamplitudes ofS and S Signal E,, is coupled to a polarized relay 112which closes one oftwo different sets of contacts depending upon thepolarity of E,,. Similarly, S and S are coupled to a second differentialamplifier lll whose output E,, is coupled to a second polarized relay113.

When signals 5,, S S and 5 are such that E and E,, are both positive,relay contacts 114 and US are closed in the E,, and E f positions,energizing motor 116 in a first polarity. This causes motor rotation ina corresponding direction which moves the contact arm of potentiometer117 in a manner to readjust the focusing current through the lensheating elements E20 and 112i. Proper focusing is indicated when thepolarity of either 5,, or E,, reverses. This causes one of the relaycontacts to switch. opencircuiting the motor circuit. Similarly. if E,,and E,, are both negative, relay contacts 114 and 115 are closed in theE,,' and E,, positions, energizing motor 116 in the opposite polarityand. thereby, causing motor rotation in the opposite sense.

The arrangement of resistors R and R/2 are required because of the powerdistribution among the sensor elements explained hereinabove.

In the two illustrative embodiments described hereinabove, thephotosensors are located along the inner surface of the waveguideenclosure. Alternatively, they can be separately fabricated on a planarsubstrate and inserted into the enclosure. or on different substratesand arranged in a staggered configuration to reduce crosstalk.Basically, all that is necessary is that they be in a position tointercept a portion of the scattered wave energy. Thus, theabove-described arrange ments are illustrative of but a small numberofthe many possi ble specific embodiments which can representapplications of the principles of the invention. Numerous and variedother arrangements can readily be devised in accordance with theseprinciples by those skilled in the art without departing from the spiritand scope ofthe invention.

We claim:

1. In an optical beam waveguide including a plurality oflenseslongitudinally spaced therealong, beam position and beam width sensingmeans comprising:

means comprising a thin thread extending transversely across saidwaveguide for scattering a small fraction of less than 1 percent of thewave energy incident thereon; and

a photosensor for measuring the intensity distribution of said scatteredwave energy.

2. The waveguide according to claim l wherein said scattering meanscomprises two threads oriented along two mutually perpendiculardirections transverse to the direction of beam propagation; and

wherein a separate photosensor measures the wave energy scattered byeach of said mutually perpendicular scattering means.

3. The sensing means according to claim 1 wherein said photosensorcomprises three photosensitive elements.

4. The sensing means according to claim 3 wherein said elements areproportioned to intercept equal amounts of the wave energy scatteredfrom a properly oriented and properly focused beam.

5. The sensing means according to claim 1 including means for refocusingand for repositioning said beam in response to said photosensormeasurements.

6. The sensing means according 'to claim 1 wherein said photosensorcomprises four photosensitive elements.

'7. The sensing means according to claim 6 wherein the two outer of saidfour elements are proportioned to intercept onethird of the energyscattered from a a properly oriented and properly focused beam; and

wherein each of the two inner of said four elements is proportioned tointercept one-sixth ofsaid scattered energy.

8. The sensing means according to claim 7 wherein the signals producedby said two inner elements are used to reposition the beam; and

wherein the signals produced in response to the scattered energyintercepted by the outer elements and the respective adjacent innerelements are used to refocus the beam.

9. The sensing means according to claim 8 wherein said beam is refocusedwhen the two differences S,-S and 8 -5 have the same polarity, where S,and S are the signals produced in response to the scattered energyintercepted by the two outer elements and S and S are the signalsproduced in response to the scattered energy intercepted by the adjacentinner elements.

1. In an optical beam waveguide including a plurality of lenseslongitudinally spaced therealong, beam position and beam width sensingmeans comprising: means comprising a thin thread extending transverselyacross said waveguide for scattering a small fraction of less than 1percent of the wave energy incident thereon; and a photosensor formeasuring the intensity distribution of said scattered wave energy. 2.The waveguide according to claim 1 wherein said scattering meanscomprises two threads oriented along two mutually perpendiculardirections transverse to the direction of beam propagation; and whereina separate photosensor measures the wave energy scattered by each ofsaid mutually perpendicular scattering means.
 3. The sensing meansaccording to claim 1 wherein said photosensor comprises threephotosensitive elements.
 4. The sensing means according to claim 3wherein said elements are proportioned to intercept equal amounts of thewave energy scattered from a properly oriented and properly focusedbeam.
 5. The sensing means according to claim 1 including means forrefocusing and for repositioning said beam in response to saidphotosensor measurements.
 6. The sensing means according to claim 1wherein said photosensor comprises four photosensitive elements.
 7. Thesensing means according to claim 6 wherein the two outer of said fourelements are proportioned to intercept one-third of the energy scatteredfrom a a properly oriented and properly focused beam; and wherein eachof the two inner of said four elements is proportioned to interceptone-sixth of said scattered energy.
 8. The sensing means according toclaim 7 wherein the signals produced by said two inner elements are usedto reposition the beam; and wherein the signals produced in response tothe scattered energy intercepted by the outer elements and therespective adjacent inner elements are used to refocus the beam.
 9. Thesensing means according to claim 8 wherein said beam is refocused whenthe two differences S1-S3 and S2-S4 have the same polarity, where S1 andS2 are the signals produced in response to the scattered energyintercepted by the two outer elements and S3 and S4 are the signalsproduced in response to the scattered energy intercepted by the adjacentinner elements.